The present invention relates broadly to the class of electrical components encompassing variable resistive devices (i.e., rheostats and potentiometers). More particularly, it relates to a contact assembly particularly adapted for a rotary potentiometer having a resistive element formed of a thick or thin film composition, such as cermet, conductive plastic or metal deposition, but which may also be adapted for a device with a wirewound resistive element.
Miniature, single-turn, rotary potentiometers typically employ a resistive element that is formed on a substrate in a substantially annular configuration, and concentrically surrounding a central conductive collector. Electrical contact is established between the collector and a selectable position on the resistive element by means of a contact mounted on a rotor. A common type of contact in such potentiometers comprises a multi-wire spring contact, cantilever-mounted on the rotor so as to brush against, or "wipe" the resistive element as the rotor is turned. Examples of this type of contact are disclosed in U.S. Pat. No. 3,704,436 to Froebe et al.; U.S. Pat. No. 4,186,483 to Laube et al.; and U.S. Pat. No. 4,427,966 to Gratzinger et al., all commonly assigned to the assignee of the subject application.
A representative example of such a prior art multi-wire contact is shown in FIGS. 15 and 16 of the present disclosure. These figures show a multi-wire contact 10, of the cantilever type, comprising an array of wires in side-by-side relationship, forming a flat, rotor-mounted body portion 12, and an unsupported portion which is bent to form a plurality of resilient, cantilevered contact fingers 14, each with a protuberance 16 near its free end to provide a contact point 16. As shown in FIG. 16, the contact may be split into two sets of fingers 14 separated by a gap 18. The larger set of fingers 14 wipes or brushes against a resistive element 20 on the substrate 22 of a potentiometer 24 (FIG. 12), while the smaller set contacts a central conductor or collector element 26. Advantageously, the fingers at the outer margins of each set are of broader cross-section than the inner fingers, as shown in FIG. 16, to form bracing fingers 28 that minimize the side-to-side movement ("chattering") of the fingers as they traverse the resistive element 20, while also minimizing any lateral spreading between the fingers 14.
Multi-wire contacts, of the type described above, exhibit low contact resistance and low contact resistance variation (CRV). Nevertheless, this type of contact still poses some problems. For example, as shown by the broken outline in FIG. 12, the contact 10 is oriented transversely in the potentiometer 24 so that its contact points 16 are substantially aligned with the radius of curvature of the resistive element 20, describing a path that is substantially tangential thereto as the contact is rotated. This orientation requires the contact 10 to occupy space within the potentiometer that must be accommodated by a clearance on the substrate 22 beyond the outer radius of the resistive element 20, thereby both wasting space on the substrate and limiting the radius of the contact's path. The result is a shorter resistive element length than might otherwise be accommodated on a given area of substrate. If the substrate area is to be kept within a specific limit due to physical constraints imposed by the application of the potentiometer, the resulting limitation on the length of the resistive element may lead to poor resolution, stability, settability, and heat dissipation, and increased contact resistance and CRV. Conversely, if the operational parameters of the component are optimized, its physical dimensions may have to be increased, thereby possibly limiting its range of applications. In short, the typical cantilever-type contact geometry requires a trade-off between size and performance, particularly in the exhibited CRV.
Another drawback to the cantilevered multi-wire contact is the display of "voltage ratio shift" (VRS), as a result of wear on the contact points 16, as illustrated in FIGS. 17a, 17b, and 17c. In those figures, it can be seen that as the contacts wear, a flat spot or "footprint" 32, of gradually increasing size, develops on the underside of the contact points 16. (The relative size of the footprint 32 shown in the drawings is exaggerated for clarity). The development of a significant footprint 32 produces what is known as "heel-and-toe" effect, which is a shift of the specific point of contact between the resistance element 20 and the contact element 10 as a result of even minute changes in the axial loading applied to the rotor and the contact element. For example, FIG. 17b illustrates the nominal position of the contact element 10 with respect to the resistance element. A small increase in the load placed on the contact element (FIG. 17a), or a slight decrease in the loading (FIG. 17c) shifts the point of contact away from its nominal position. This shifting contact point, in turn, produces the VRS effect.
Still another problem with the prior art multi-wire contact 10 is the aforementioned "chattering" effect. Although this effect is reduced by the bracing fingers 28 (FIG. 16), there is still some side-to-side displacement of the contact fingers as indicated by the broken outline 34 in FIG. 16. Like the "heel-and-toe" effect, the chattering results in an increase in VRS, and it can also produce changes in the resistance setting. A related problem is that of the spreading or splaying of the fingers 14, again minimized, but not eliminated, by the bracing fingers 28. This, too, can result in degraded performance characteristics.
In an attempt to overcome these problems with the multi-wire contact, a single conductor contact has been devised, as shown in FIGS. 9, 10, and 11. This single conductor contact, indicated by the numeral 40, comprises a substantially annular metal stamping having a solid portion or base 42 that is attached to the rotor of a rotary-action variable resistive device. A substantial portion (about one-half to two-thirds) of the circumference of the stamping is divided by arcuate cuts into a plurality of substantially concentric arcuate contact elements 44. These contact elements 44 are raised out of the plane of the base portion 42, as shown in FIGS. 10 and 11, and each of them has a "hump" or protuberance 46 at the midpoint of its arc length. These protuberances 46 provide the contact points that brush or wipe against the resistive element. In some applications, an inner contact element 48, adjacent the central aperture 50 of the stamped contact 40, may function as a collector contact. Most applications, however, where the stamped contact 40 is used, are of the so-called "hot rotor" design where the rotor itself (not shown in the drawing in connection with the stamped contact 40) extends through the aperture 50 and establishes electrical contact with the collector.
An advantage of the stamped, single conductor contact 40 is that it is not as prone to chattering or splaying as the multi-wire contact 10. Moreover, its annular configuration allows a more efficient use of space within the potentiometer, requiring less clearance around the outside of the resistive element, and thereby allowing a longer resistive element to be used. This is illustrated in FIG. 14, where a potentiometer 24', having a substrate 22', includes an annular contact that is indicated diagramatically by the broken circle 52. It can be seen that the potentiometer 24' can be provided with a much longer resistive element 20', as compared with the resistive element 20 used with the multi-wire contact 10. (See FIG. 12.) This arrangement thus overcomes the previously-discussed disadvantages associated with the cramped resistive element configuration shown in FIG. 12.
In addition, the stamped contact 40 is less likely to exhibit VRS as a result of the previously-discussed "heel-and-toe" effect. This, along with the aforementioned reduction in "chattering", gives the stamped contact 40 very favorable VRS characteristics as compared to the multi-wire contact 10.
Furthermore, the concentric contact elements 44 of the stamped contact provide a longer spring arm than do the fingers 14 of the multi-wire contact 10. This allows the use of a lower spring rate which, in turn, results in a lower probability of the contact taking a "set" under load, with a resultant possible degredation of contact performance characteristics when the load is varied.
Nevertheless, the stamped contact 40 typically exhibits higher contact resistance and CRV than does the multi-wire contact 10, due to the smaller number of discrete contact elements 44 compared to the number of contact fingers 14 employed in the multi-wire contact. Limitations in manufacturing techniques for the stamped contact 40 simply do not allow the fabrication of contact elements 44 that are as narrow as the thin wires used in the multi-wire contact.
Thus, it can be seen that there has been a long-felt, but as yet unsatisfied need, for a contact assembly for a rotary-action variable resistor that combines the aforementioned advantages of both the multi-wire contact and the single conductor stamped contact, and yet which avoids or minimizes the disadvantages associated with each of these designs.